# cubic polynomial

• Mar 22nd 2008, 08:44 AM
gracey
cubic polynomial
the cubic polynomial x^3 +ax^2 + bx - 8 where a and b are constants has factors x+1 and x+2. find values a and b

i'm thinking you expand (x+2)(x+1) to get the quotient of x^2 + 3x + 2 but i'm not sure

would be so grateful for some help thanks (Speechless)
• Mar 22nd 2008, 08:53 AM
topsquark
Quote:

Originally Posted by gracey
the cubic polynomial x^3 +ax^2 + bx - 8 where a and b are constants has factors x+1 and x+2. find values a and b

i'm thinking you expand (x+2)(x+1) to get the quotient of x^2 + 3x + 2 but i'm not sure

would be so grateful for some help thanks (Speechless)

Let the third factor be (x + c).

Then
$(x + 1)(x + 2)(x + c) = (x^2 + 3x + 2)(x + c) = x^3 + (c + 3)x^2 + (3c + 2)x + 2c$

Match up the coefficients.

-Dan